The never ending trend toward smaller electronic devices increases the pressure to develop smaller and more efficient components. For example, wireless communications systems are requiring increasingly enhanced performance from passive components used for signal processing, particularly those operating above one GHz. In the case of SAW filters, characteristics typically demanded include low insertion loss, low passband ripple, high degree of linearity of phase and high selectivity. To meet these demands single-phase unidirectional transducers (SPUDTs) are frequently used. SPUDT devices can also be used for SAW sensors and SAW radio frequency identification tags.
A SPUDT structure calls for the placement of reflectors and transducers in such a way that, within each unit cell, the center of transduction is shifted with respect to the center of reflection. Ideally, this phase shift should be equal to ±one-half of pi (±π/2). In most SPUDT structures, electrodes one-eighth of a Rayleigh SAW wavelength wide and reflectors ranging from one-fourth to three-eighths of a wavelength wide are used to obtain a nonreflecting transduction. In the majority of cases the electrodes are one-eighth of a wavelength or narrower. Consequently, in the GHz range the critical dimensions of electrodes are beyond the limits of feasibility for large scale fabrication techniques based on optical lithography.
For SAW devices operating at 2 GHz and higher frequencies, the wavelength is about 2 μm. Thus, an electrode one-eighth of a wavelength wide has an absolute width of about 0.25 μm. With the thickness ranging from 2% to 10% of a wavelength, the absolute height of the electrode is about 40–200 nm. This small aluminum cross section for the electrode causes resistive losses to become unacceptably high. For this reason, SPUDT transducers are seldom used above 1 Ghz.
Accordingly, what is needed in the art is a low-loss unidirectional transducer that can operate on a substrate at frequencies higher than 1 GHz that can be manufactured utilizing large scale fabrication techniques based on optical lithography.
Also needed in the art are better reflector configurations to use with SAW radio frequency identification tags. In the case of SAW identification tags, it is important that as much of the energy reflected in response to a transducer generated interrogation pulse be captured as possible. If an aluminum reflector located on a substrate is the same size as the transducer and if that reflector is straight and substantially perpendicular to the interrogation pulse, a substantial amount of energy generated by the transducer is not going to be included in the reflected pulse. This is because a portion of the pulse generated by the transducer does not impact a reflector due to the fact that it expands in size as it travels down the SAW tag surface away from the transducer.
Thus, what is needed in the art is a better reflector for use on a SAW tags that have the capability of capturing more of the interrogation pulse energy in order to return a more vigorous reflected signal to the transducer.